PIC Tutorial - Stepper Board

Stepper Board

This is the Stepper Board, it contains four transistor drivers for feeding a uni-polar stepper motor, such as those commonly found in old 5.25 inch floppy drives and printers. The transistors are fed via 470 ohm resistors, to limit the base current, and the eight diodes are there to absorb the back EMF (Electro Motive Force) from the stepper motor coils when they turn off - when you turn an inductor off, be it a motor or relay, it generates a high reverse voltage spike (which is how car ignition coils work) - this would be likely to damage the transistors, the diodes prevent it. The stepper motor itself consists of two centre-tapped windings, with the centre-taps connected to the positive supply rail, to make the motor move you need to activate the coils in a certain sequence, it then moves one step for each value in the sequence - a half-stepping mode is also available, which gives double the number of steps by using a slightly different sequence. The wiring of the 6 pin connector to the motor is how the two motors I have are wired, one was from an old floppy, the other is an Epson printer motor - both are wired the same way on similar 6 pin connectors - not all motors are likely to be wired exactly the same though!. I've marked the connections to the motor as A, B, C and D, plus two marked '+', these are connected to the positive supply, and to make the motor step the other four are grounded in a particular sequence. The simplest sequence is to select them one at a time, A, B, C then D, this produces 4 steps in a clockwise direction - to produce one full rotation this needs to be repeated 50 times as my particular motor has 200 steps per revolution, giving 1.8 degrees resolution (in full step mode).

The jumper J1 can be used to feed the motor from the 5V supply, and this should be fine just for experimenting, but the motors are designed for 12V, and won't produce much power from 5V - by disconnecting J1 you can feed a 12V supply to the top of D5, D6, D7 and D8 - obviously if you don't disconnect J1 you are very likely to fry the PIC!.

This is a top view of the Stepper Board, it consists of four transistors, with base feed resistors, and eight diodes - two of which are mounted the opposite way round - these are the two nearest the motor connector!. 
A bottom view of the Stepper Board, the sixteen track cuts are marked with blue circles, and it has ten wire links on the top, plus it has an optional extra track cut (between holes) for jumper J1 - this isn't shown in the picture as I didn't install it, but looking at this picture it's the second track down and anywhere between 5-12 holes from the left, with the external power fed in to the left of the cut.

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